Relationship of close contact settings with transmission and infection during the SARS-CoV-2 Omicron BA.2 epidemic in Shanghai

Introduction We analysed case-contact clusters during the Omicron BA.2 epidemic in Shanghai to assess the risk of infection of contacts in different settings and to evaluate the effect of demographic factors on the association of infectivity and susceptibility to the Omicron variant. Methods Data on the settings and frequency of contact, demographic characteristics and comorbidities of index cases, contacts and secondary cases were analysed. Independent effect of multiple variables on the risk for transmission and infection was evaluated using generalised estimating equations. Results From 1 March to 1 June 2022, we identified 450 770 close contacts of 90 885 index cases. The risk for infection was greater for contacts in farmers’ markets (fixed locations where farmers gather to sell products, adjusted OR (aOR): 3.62; 95% CI 2.87 to 4.55) and households (aOR: 2.68; 95% CI 2.15 to 3.35). Children (0–4 years) and elderly adults (60 years and above) had higher risk for infection and transmission. During the course of the epidemic, the risk for infection and transmission in different age groups initially increased, and then decreased on about 21 April (17th day of citywide home quarantine). Compared with medical workers (reference, aOR: 1.00), unemployed contacts (aOR: 1.77; 95% CI 1.53 to 2.04) and preschoolers (aOR: 1.61; 95% CI 1.26 to 2.05) had the highest risk for infection; delivery workers (aOR: 1.90, 95% CI 1.51 to 2.40) and public service workers (aOR: 1.85; 95% CI 1.64 to 2.10) had the highest risk for transmission. Contacts who had comorbidities (aOR: 1.10; 95% CI 1.09 to 1.12) had a higher risk for infection, particularly those with lung diseases or immune deficiency. Conclusion Farmers’ markets and households were the main setting for transmission of Omicron. Children, the elderly, delivery workers and public service workers had the highest risk for transmission and infection. These findings should be considered when implementing targeted interventions.


This file includes:
Figure S1 to Figure S3.Table S1 to Table S21.
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Table S14 .
Statistical summary of generalized estimating equations used to estimate risk for infection with different contact frequency in the Omicron epidemic in Shanghai in 2022................................... 25BMJ Publishing Group Limited (BMJ) disclaims all liability and responsibility arising from any reliance Supplemental material placed on this supplemental material which has been supplied by the author(s)

Table S21 .
Statistical summary of generalized estimating equations used to estimate risk for infection by contacts in other settings in the Omicron epidemic in Shanghai in 2022............................................... 34BMJ Publishing Group Limited (BMJ) disclaims all liability and responsibility arising from any reliance Supplemental material placed on this supplemental material which has been supplied by the author(s) BMJ Publishing Group Limited (BMJ) disclaims all liability and responsibility arising from any reliance Supplemental material placed on this supplemental material which has been supplied by the author(s) BMJ Glob Health doi: 10.1136/bmjgh-2023-012289 :e012289.8 2023; BMJ Glob Health , et al.Zhu W BMJ Publishing Group Limited (BMJ) disclaims all liability and responsibility arising from any reliance Supplemental material placed on this supplemental material which has been supplied by the author(s) BMJ Glob Health doi: 10.1136/bmjgh-2023-012289 :e012289.8 2023; BMJ Glob Health , et al.Zhu W

Table S1 . Occupations of index cases, close contacts, and secondary cases reported during the Omicron epidemic in Shanghai from 1 March to 1 June, 2022.
*

Table S3 . Characteristics of secondary cases stratified by whether secondary cases themselves reported symptoms.* Characteristics Overall Self-reported symptoms
*Numbers are given as n (%) or median (P25, P75).BMJ Publishing Group Limited (BMJ) disclaims all liability and responsibility arising from any reliance Supplemental material placed on this supplemental material which has been supplied by the author(s) BMJ Glob Health doi: 10.1136/bmjgh-2023-012289

Table S4 . Characteristics of index cases, close contacts, and secondary cases reported during the Omicron epidemic in Shanghai in Phase 1 (1 to 27 March 2022).
*Numbers are given as n (%) or median (P25, P75).BMJ Publishing Group Limited (BMJ) disclaims all liability and responsibility arising from any reliance Supplemental material placed on this supplemental material which has been supplied by the author(s) BMJ Glob Health doi: 10.1136/bmjgh-2023-012289

Table S8 . Age-specific secondary attack rate, risk for infection among close contacts and risk for transmission among index cases during the Omicron epidemic in Shanghai in 2022.
"Unknown" for each characteristic was excluded from the analysis.† Adjusted for sex, occupation, comorbidities, and vaccination status of index cases and close contacts; and for clinical severity, self-reported symptoms and time from positive test to isolation of index cases. *

Table S9 . Risk for secondary infection of contacts in different settings during the four phases of the Omicron epidemic in Shanghai in 2022.
* "Unknown" for exposure settings was excluded from the analysis.† Adjusted for age, sex, comorbidities, and vaccination status of index cases and close contacts; and for selfreported symptoms of index cases.

Table S11 . Secondary attack rate (SAR) and risk for transmission by index cases with different comorbidities during the Omicron epidemic in Shanghai in 2022.
Adjusted for age group, sex, occupation, and vaccination status of index cases and close contacts; for comorbidities of close contacts; and for clinical severity and self-reported symptoms in index cases.

Vaccination status of index cases
BMJ Publishing Group Limited (BMJ) disclaims all liability and responsibility arising from any reliance Supplemental material placed on this supplemental material which has been supplied by the author(s) "Unknown" for each characteristic was excluded from the analysis.BMJ Publishing Group Limited (BMJ) disclaims all liability and responsibility arising from any reliance Supplemental material placed on this supplemental material which has been supplied by the author(s) *

Vaccination status of index cases
Publishing Group Limited (BMJ) disclaims all liability and responsibility arising from any reliance Supplemental material placed on this supplemental material which has been supplied by the author(s) BMJ